1. Field of the Invention
The present invention relates to an illumination device having a cold cathode fluorescent tube, and a display device including the illumination device.
2. Description of the Related Art
In liquid crystal display devices such as those for use in on-vehicle navigators, on-vehicle televisions and on-vehicle meters, direct backlights and edge light illumination devices have been widely used. A cold cathode fluorescent tube is used as a light source of such illumination devices for the liquid crystal display devices. The cold cathode florescent tube has advantages over an incandescent lamp such as excellent luminous efficacy, a lesser amount of heat generation, a longer life, and superior luminance (luminous flux) distribution. Moreover, the cold cathode fluorescent can be formed as a thin element.
However, a conventional cold cathode fluorescent tube which has been generally used has a disadvantage that the characteristics thereof are affected by the temperature at which the cold cathode fluorescent tube is used. This results from the fact that the characteristics of the conventional cold cathode fluorescent tube depend on the vapor pressure of mercury which fills the tube. A luminance (luminous flux) rising characteristic (i.e., a "start-up" characteristic) at a low temperature and luminance at a low temperature are most seriously affected. For example, on-vehicle illumination devices may be used at a broad range of temperatures from about 80.degree. C. to about -30.degree. C. (from the tropics to the Polar Regions). The above-mentioned conventional cold cathode fluorescent tube has maximum luminous efficacy at an ambient temperature of about 40.degree. C., and therefore, can be practically used without any problems at a temperature between about 5.degree. C. to about 40.degree. C. However, when used at a low temperature close to -30.degree. C., the conventional cold cathode fluorescent tube might require a long time to achieve prescribed luminance, or might easily fail to start.
In order to facilitate the rise of the luminance at a low temperature as well as to improve the luminance at a low temperature, Japanese Laid-Open Publication No. 63-224140 discloses a structure in which an exothermic body which self-controls its temperature is provided around a cold cathode fluorescent tube so as to increase a surface temperature of the cold cathode fluorescent tube. In addition, Japanese Laid-Open Publication No. 7-43680 discloses a structure in which a heater for heating a cold cathode fluorescent tube is provided. Power supplied to the heater is controlled by continuous measuring of a surface temperature of the cold cathode fluorescent tube by a temperature detection element and a temperature detection circuit, thereby effecting control of a heater power supply and an inverter power supply.
More specifically, the above-mentioned conventional example employs a method for controlling power supplied to the heater so as to render the cold cathode fluorescent tube stable in a saturation temperature range (i.e., stable in a temperature environment).
Moreover, a method for increasing a current applied to a cold cathode fluorescent tube only during start-up so as to improve the rise of luminance at a low temperature has also been proposed. For example, Japanese Laid-Open Publication No. 61-74298 discloses a structure in which control means increases a current applied to a cold cathode fluorescent tube to a value larger than a rated value only for a prescribed period from the start to completion of the rise of luminance.
In addition, Japanese Laid-Open Publication No. 59-60880 discloses a method for increasing an interrupting current for a switching circuit for a prescribed period from activation so as to increase an energy of the fluorescent tube.
However, the above-mentioned conventional examples have the following problems.
In the case where such an exothermic body or a heater is used to heat a cold cathode fluorescent tube, large luminous flux losses will occur, and therefore, the amount of illumination light will be reduced. Such luminous flux losses occur because the exothermic body or the heater itself is in close contact with a surface of the cold cathode fluorescent tube and thus blocks the luminous flux of the cold cathode fluorescent tube. Moreover, should a control circuit for the heater malfunction, the heater would continue to generate heat. Furthermore, the heater itself and its associated parts including a control circuit, would be additionally required, causing a significant increase in the manufacturing cost. Moreover, additional power (typically, several tens of watts) required for the heater would impose a load to the battery as well as affect the vehicle itself when, for example, the on-vehicle illumination device is started. Especially in winter, since a battery temperature may be below 0.degree. C., such a load to the battery and an influence on the vehicle can not be ignored.
In the case where the above-mentioned method for increasing a current applied to the cold cathode fluo- rescent tube for a prescribed period from activation so as to facilitate start-up at a low temperature is used, a current larger than a rated value is applied to the cold cathode fluorescent tube upon activation, and the cold cathode fluorescent tube could be damaged seriously. Therefore, a life of the cold cathode fluorescent tube would be reduced. Moreover, this method does not sufficiently improve the rise of the luminance at a low temperature as compared to the above-mentioned method of using the heater. Therefore, this method is often used together with the method of using the heater.
Consequently, there is a demand for the development of display devices such as a liquid crystal display device using a cold cathode fluorescent lamp as a light source, which can provide required luminance even when the display devices are used in a broad temperature range from about 80.degree. C. to about -30.degree. C. (i.e., from the tropics to the Polar Regions).